Up to date, to improve impact strength of thermoplastic polyester resin, studies have been undergoing on thermoplastic polyester resin compositions composed of thermoplastic polyester resin and thermoplastic graft copolymer, the impact strength modifier. However, as impact strength increased, other physical properties decreased. For example, U.S. Pat. No. 4,117,034 and No. 4,180,494 describe a method to improve impact strength of a polyester resin composition by adding a core-shell type polymer containing butadiene-based core to thermoplastic polyester resin as an impact strength modifier. However, this method has a problem of damaging transparency of the polyester resin composition.
As an effort to retain transparency of polyester resin, U.S. Pat. No. 5,231,056 and No. 5,409,967 describe a method to increase impact strength of a polyester resin composition by adding an impact modifier in which shell is composed of styrene copolymer to polyester resin. According to this method, transparency of the polyester resin composition was preserved but impact strength was not improved satisfactorily.
Korean Patent Publication Nos. 2002-0060011, 2003-0018671 and 2007-0021894 describe a method of using rubber latex having multi-layered structure for graft copolymer in order to improve impact strength of polychlorinated resin. However, when this method was applied on a thermoplastic polyester resin composition, transparency was decreased because of difference of refraction index.